The present invention relates to a semiconductor device and a fabrication method thereof, and particularly to a semiconductor device in which an n-channel field effect transistor and a p-channel field effect transistor are provided on a common base-substrate, and a fabrication method thereof.
In recent years, along with rapid advance in digitization of information processing means, there have been strong demands to increase the operational speeds of semiconductor devices and to reduce the power consumption of semiconductor devices. Conventionally, the increase in operational speed of semiconductor devices has been achieved by making finer device structures, and the reduction in power consumption thereof has been achieved by adopting semiconductor devices of a complementary type including n-channel and p-channel field effect transistors. The technique for making finer device structures, however, has already reached a limitation of a lithography process, in which a pattern having a size smaller than a wavelength of exposure light has been needed to be formed by lithography, and accordingly, at the lithography step, it has come to be difficult to ensure a sufficient process tolerance. In other words, the attempt to increase the operational speeds of semiconductor devices by making finer device structures is coming closer to a limitation.
For field effect transistors, there has been proposed an attempt to solve the above problem by imparting a strain effect to a channel layer. The formation of devices using a material layer having such a strain effect has become possible by an advanced thin film formation technology using a group IV element containing semiconductor material such as silicon or a silicon-germanium compound, and at present, improved low voltage devices have been actively developed by using material layers having the strain effect.
The strain effect means a phenomenon that if stress is applied to a semiconductor thin film, the energy band of the semiconductor is strained, to thereby change the effective masses of carriers in the semiconductor. The formation of a semiconductor thin film having the strain effect has become possible by controlling an internal stress applied to a multi-layer film typically containing a silicon layer and a silicon-germanium compound layer by a molecular beam epitaxy process or an ultrahigh vacuum chemical vapor deposition process (UHV-CVD). High performance MOS based devices, sensors, and the like of a type in which a difference in band gap and film strain are controlled by hetero-junction have been also developed.
A silicon based MOS (Metal-Oxide-Semiconductor) including layers each having a strain effect can be produced by sequentially stacking a buffer layer made from a silicon-germanium compound having a germanium concentration gradually increased toward an upper surface of the buffer layer, a relax layer made from a silicon-germanium compound, and a silicon layer on a silicon substrate in this order. In this MOS transistor, since tensile stress is generated in the silicon layer, a strain effect due to the tensile stress appears, with a result that the mobility of electrons is increased by the strain effect in the silicon layer; and since compressive stress is generated in the silicon-germanium compound layer formed on the silicon substrate, a strain effect due to the compressive stress appears, with a result that the mobility of positive holes is increased by the strain effect in the silicon-germanium compound layer.
A field effect transistor produced by controlling stress in a channel layer by making use of the above-described strain effect exhibits a high mutual conductance [gm (mobility)]. A pMOS transistor produced by making use of the strain effect has been disclosed in Appl. Phys. Letter (USA), 63(1993) S. P. Voinigensen et al., p660, and IEEE Electronic Devices (USA), 43(1996), L. H. Jiang and R. G. Elliman, p97. Further, an nMOS transistor produced by making use of the strain effect has been disclosed in Appl. Phys. Letter (USA), 64(1994) K. Ismail et al., p3124 and IEDM 94-37 (USA), (1994) J. Welser et al.
The semiconductor devices produced by making use of the strain effect have the following problem:
In a silicon layer formed on a relax layer, the mobility of electrons is increased by the strain effect due to tensile stress; however, the mobility of positive holes is reduced. Accordingly, while an nMOS transistor having an improved low voltage can be obtained, the performance of a pMOS transistor cannot be improved, with a result that the performance of the CMOS cannot be significantly improved.
On the other hand, in a silicon-germanium compound layer formed on a silicon substrate, the mobility of positive holes is improved by the strain effect due to compressive stress; however, the mobility of electrons is reduced. Accordingly, while a pMOS transistor having an improved low voltage can be obtained, the performance of an nMOS transistor cannot be improved, with a result that the performance of the CMOS cannot be significantly improved.
As described above, according to the related art method, it has been impossible to obtain a high performance CMOS and hence to produce a semiconductor device having a high performance and a low power consumption.